In the "Temperature control" article it is stated that this unit will not work with RPM monitoring. Is the reason behind this the fact that it "chops" the GND wire, not the +12V one? That would explain why one would need a P channel fet so we can chop another wire. (A quick google search did not reveal how the RPM monitoring works, but I am guessing that it probably sends some impulses against the GND)
What would be the scheme if we would want it to work with the monitoring? Could it basically be reversed or there is something more to it? Maybe author of the article could update it with the other scheme too, maybe with not that detailed instructions, just the scheme...